The present invention relates in general to a hybrid propulsion system for a motor vehicle comprising an internal combustion engine, an electric machine, and a gearbox, wherein the electric machine is kinematically connected to the gearbox by means of a first drive transmission mechanism comprising a pinion carried by the output shaft of the electric machine, one of the gears of the gearbox and an intermediate gear, wherein the gearbox comprises a gear control device and a first shaft and a second shaft on which are mounted a plurality of idle gears and a plurality of slidable engagement sleeves each of which is associated with one or two idle gears to connect them for rotation with the respective first or second shaft, wherein the gear control device comprises a first drum control device associated with the first shaft to control the movement of the slidable engagement sleeves disposed on that shaft, a second drum control device associated with the second shaft to control the movement of the slidable engagement sleeves disposed on that shaft, and an actuation unit which is arranged to bring about rotation of the first and second drum control devices in a synchronized manner, wherein the actuation unit comprises a motor device and a second drive transmission mechanism, wherein the second drive transmission mechanism comprises a first gear associated with the first drum control device, a second gear associated with the second drum control device, and an intermediate gear, and wherein the first gear meshes directly or indirectly with an output pinion of the motor device and the intermediate gear meshes directly both with the first gear and with the second gear. More particularly, the invention relates to the arrangement of the intermediate gear of the above-defined second drive transmission mechanism.